Field of the Invention
The present invention relates to a polygon mirror serving as a rotor in a polygon mirror scanning motor.
Prior Art
FIG. 2 is a sectional view of such a rotor in a conventional polygon mirror scanning motor. In FIG. 2, reference numeral 1 denotes a ceramic ring. A yoke 2 is shrink-fitted onto the ceramic ring 1. Then, a higher degree of planeness and surface roughness are imparted to the surfaces of the yoke 2 and a yoke 5 which are to contact a mirror surface formation member 3 defining a mirrored surface 4. The mirror surface formation member 3 is firmly clamped between the yoke 2 and the yoke 5 with a plurality of screws 16. Furthermore, a rotor magnet 6 having a plurality of magnetic poles of the motor is fixedly secured to the underside of the yoke 5.
FIG. 3 is a sectional view of a polygon mirror scanner motor employing the rotor shown in FIG. 2. A supporting shaft 8 fixed to a mount 7 extends through the ceramic ring 1. A radial bearing member 9 is firmly secured to the supporting shaft 8 and the ceramic ring 1. Moreover, thrust plates 10 and 11 are fixedly secured to the supporting shaft 8 so as to confront top and bottom end surfaces of the ceramic ring 1, respectively. A thrust dynamic pressure bearing is defined by the top and bottom end surfaces of the ceramic ring 1 and the respective confronting surfaces of the thrust plates 10 and 11. A radial dynamic pressure bearing is defined by the inner periphery of the ceramic ring 1 and the outer periphery of the radial bearing member 9. Reference numeral 12 designates a fixing bolt for fixing the thrust plates 10 and 11, the radial bearing member 9, and mounting plate 13 to the supporting shaft 8. A stator coil 14 is arranged on the top surface of the mount 7 so as to confront the rotor magnet 6. Reference numeral 15 indicates a cover.
In the above-described polygon mirror scanner motor, when the stator coil is sequentially energized, the inner periphery of the ceramic ring 1 of the rotor is supported on the radial bearing member 9, and the top and bottom end surfaces of the rotor are supported on the confronting surfaces of the thrust plates 10 and 11 for rotation. It is to be noted that the supporting shaft 8 and the radial bearing member 9 are collectively referred to as a fixing shaft, although the supporting shaft 8 may solely (without the radial bearing member 9) comprise the fixing shaft. In this case, the fixing shaft (supporting shaft) serves also as the radial bearing member.
The conventional rotor, that is, the polygon mirror described above, has the following disadvantages.
(1) Since the yoke 2 is shrink-fitted onto the ceramic ring 1, the outer diameter of the ceramic ring 1 and the inner diameter of the yoke 2 must be finished with a higher degree of roundness, concentricity, surface roughness, and dimensional tolerances, which contributes to high machining costs for the ceramic ring 1 and the yoke 2.
(2) Furthermore, after the shrink-fitting of the yoke 2 onto the ceramic ring 1, the surface of the yoke 2 in contact with the mirror surface formation member 3 must be subjected to a secondary machining operation to obtain strict planeness and surface roughness. Additionally, an additional assembly step is created by the need to thread the screw 16, securing the mirror surface formation member 3 to the yoke 5, with a predetermined controlled torque. Moreover, the rotor as a whole might be unbalanced due to looseness in the fitting section which is inevitably created in the assembly irrespective of the dimensional accuracy of the parts themselves, requiring numerous steps to correct the imbalance.
(3) Furthermore, depending on the degree to which the screw 16 is tightened, the mirror surface 4 may present a minute deformation, which in particular exerts a remarkable adverse influence, at the time of actual rotation, on the surface stability and jitter characteristic of the motor. The mirror surface formation member 3 must be provided with several through-holes for receiving several screws, thereby requiring a large number of machining steps, including the steps of processing the corners after the holes are formed. As a result, the manufacturing cost of the mirror is high.
(4) Moreover, the yoke 2 has a smaller height than the ceramic ring 1 when the shrink-fitting is performed, and hence only a portion of the ceramic ring is subjected to a shrink-fit. Thus, an uneven stress due to shrink-fitting is inevitably applied to the ceramic ring, which may possibly produce a crack in the ceramic ring 1 in an extreme case. In addition, the inner peripheral surface and the top and bottom surfaces of the ceramic ring 1 are liable to be deformed, which results in uneven thrust clearance and radial clearance between the rotor and the stator when incorporated into the scanner motor, which in turn adversely influences the rotational displacement of the motor. This is a cause of poor surface stability and jitter characteristic of the motor.